The present invention relates to a process for selectively hydrogenating natural oils, in order to reduce their content in polyunsaturated compounds while limiting the formation of solid compounds.
Some natural or vegetable oils, such as soybean oil, sunflower oil, solza oil, or corn oil, contain compounds having several double bonds (polyenic compounds), for instance three double bonds (trienic compounds), and two double bonds (dienic compounds), in admixture with compounds having only one double bond (monoenic compounds), and saturated compounds. For instance, soybean oil contains triglycerides of fatty acids comprising about 10% linolenic acid (a fatty acid containing 18 carbon atoms and three C .dbd. C double bonds), about 50% of linoleic acid (fatty acid containing 18 carbon atoms and two double bonds), about 25% of oleic acid (fatty acid containing 18 carbon atoms and one double bond), and saturated fatty acids (stearic and palmitic acids).
In order to increase the stability of these oils, it is necessary to significantly reduce the content in linolenic acid glycerides and, in part, the content in glycerides of dienic acids. This reduction of the amount of unsaturated compounds is most often carried out by hydrogenation. However, for some applications, for instance for the use of these oils as cooking oils or for preparing fatty acids for paints and coatings, the formation of solid products must be obviated or at least reduced. For these reasons only a partial hydrogenation must take place and this hydrogenation must be selective. It is of prime importance to limit:
(A) THE FORMATION OF SATURATED COMPOUNDS; POLYENIC COMPOUNDS MUST THEREFORE BE HYDROGENATED TO DIENIC AND MONOENIC COMPOUNDS;
(B) The isomerization of cis- to trans-isomers; however, isomerization occurs always during hydrogenation and this isomerization results in the formation of solid products, since the trans-isomers have a higher melting point than cis-isomers;
(C) THE FORMATION OF CONJUGATED DIENIC COMPOUNDS, WHICH ARE UNSTABLE.
It has already been suggested to carry out the selective hydrogenation of natural oils in the presence of copper catalysts. However, these catalysts have some drawbacks. For instance, it is necessary to avoid the presence of even traces of these catalysts in the partially hydrogenated products, since copper promotes oxidation of these products. Moreover, these copper catalysts are far less active than nickel catalysts. By contrast, hydrogenation in the presence of nickel catalysts in less selective and results in the formation of too high amounts of solid products.